Patterns of Inheritance Genetic variation Patterns of Inheritance

Learning outcomes (i) the contribution of both environmental and genetic factors to phenotypic variation To include examples of both genetic and environmental contributions – environmental examples could include diet in animals and etiolation or chlorosis in plants. (ii) how sexual reproduction can lead to genetic variation within a species Meiosis and the random fusion of gametes at fertilisation.

Key definitions Define the terms genotype and phenotype Genotype – the genetic make up of an organism Phenotype – the visible characteristics of an organism From GCSE, what influences the phenotype? Both genotype (genetic factors) and the environment

Genetic factors 3 things influence the genetic factors: Gene mutations (i) the contribution of both environmental and genetic factors to phenotypic variation Genetic factors 3 things influence the genetic factors: Gene mutations Chromosome mutations Sexual reproduction

Gene mutations (i) the contribution of both environmental and genetic factors to phenotypic variation Mutagens can influence the rate of mutation. These can include: Physical agents – X-rays, gamma rays and UV light Chemical agents – benzopyrene (in tobacco smoke), mustard gas, free radicals Biological agents – some viruses, transposons (jumping genes), food contaminants (mycotoxins from fungi) Mutations can be harmful, advantageous or neutral. Mutations that occur during gamete formation are persistent (transmitted through many generations) and random (not directed by need)

Chromosome mutations These occur during meiosis. Create a list of the different types of chromosome mutations, how they occur and the influence they may have.

(i) the contribution of both environmental and genetic factors to phenotypic variation Chromosome mutations Deletion – part of the chromosome, containing genes and regulatory sequences, is lost Inversion – sections of chromosome mat break off, turn through 180o and join again. All the genes are still present but may be too far away from their regulatory nucleotide sequence to be expressed. Translocation – a piece of chromosome breaks off and becomes attached to another chromosome. This may affect both chromosomes. Duplication – a piece of chromosome may be duplicated. Overexpression can be harmful because too many of certain proteins may disrupt metabolism.

Chromosome mutations (i) the contribution of both environmental and genetic factors to phenotypic variation Non-disjunction – one pair of chromosomes or chromatids fails to separate leaving one gamete with an extra chromosome. Eg Downs syndrome (trisomy 21) and klinefelter syndrome (an extra X chromosome XXY) Aneuploidy – the chromosome number is not an exact multiple of the haploid number e.g. trisomy Polyploidy – if a diploid gamete is fertilized by a haploid gamete, the resulting zygote will be triploid. The fusion of 2 diploid gametes leads to a tetraploid zygote. Many cultivated plants are polyploidy

(ii) how sexual reproduction can lead to genetic variation within a species How can sexual reproduction lead to genetic variation? During meiosis: Allele shuffling – swapping of alleles between sister chromatids during crossing over in prophase 1. Independent assortment – of chromosome during metaphase/anaphase 1 and 2 Random fertilisation – any male gamete can combine with any female gamete.

Environmental factors (i) the contribution of both environmental and genetic factors to phenotypic variation Some phenotypic variation is caused only by the environment and not passed on in genes: Speaking with a regional dialect Loss of limbs or scars Some phenotypic variation is caused by the environment interacting with genes: Diet in humans Chlorosis in plants (plants grown in dim light with a lack of magnesium are yellow and cannot photosynthesis). They have the genotype for making chlorophyll but environmental factors prevent them expressing the gene.

Key definitions Write definitions for the following words we will be using over the next few lessons Allele, heterozygous, homozygous, monogenic, dihybrid, codominance, multiple allele, sex-linked, autosomal linkage, epistasis

Key definitions Allele – a version of a gene Heterozygous – not true breeding; having different alleles at a particular gene locus on a pair of homologous chromosomes Homozygous - true breeding; having identical alleles at a particular gene locus on a pair of homologous chromosomes Monogenic – determined by a single gene Dihybrid – involving 2 gene loci

Key definitions Codominance – where both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype Multiple alleles – characteristic for which there are 3 or more alleles in the populations gene pool Sex-linked – genes present on one of the sex chromosomes Autosomal linkage – gene loci present on the same autosome (non sex chromosome) that are often inherited together Epistasis – interaction of non-linked gene loci where one masks the expression of the other

Learning outcomes (i) the contribution of both environmental and genetic factors to phenotypic variation To include examples of both genetic and environmental contributions – environmental examples could include diet in animals and etiolation or chlorosis in plants. (ii) how sexual reproduction can lead to genetic variation within a species Meiosis and the random fusion of gametes at fertilisation.